Push-pull amplifier circuits



I l v n/vro/vp ixr g T T BY Dec. 16, 1941. A. L. TIMMER 2,266,096

PUSH-PULL AMPLIFIER CIRCUITS Original Filed Aug. 21, 1934 ATTORNEY.

Patented Dec. 16, 1941 PUSH-PULL AMPLIFIER CIRCUITS Anton Leendert Timmer, Eindhoven, Netherlands, assignor, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Continuation of application Serial No. 740,778, August 21, 1934.- This application May 26, 1939, Serial No. 275,836. In Great Britain September 23, 1933 1 Claim.

This invention relates to electric valve amplifiers, and more especially to amplifiers of the socalled push-pull, or balanced, type.

This application is substituted for my copending application Serial No. 740,778, filed August 21, 1934.

In amplifiers of this class it has been customary to so adjust the grid bias that the valves arranged in balance are working on the straight parts of their anode current-grid voltage characteristics. This arrangement presents the advantage that the direct current fiowing in the anode circuits is substantially independent of the amplitude of the oscillations impressed on the grids, but has the disadvantage that distortion will occur when said amplitude exceeds a certain value, which is comparatively low. Amplifiers of this type are generally called class A amplifiers.

It has also been customary, mainly in the case of battery-fed sets, in order to increase the life of the anode battery, to make the balance connected valves work at or very near the point of their characteristic, where the anode current is cut off (class B amplifiers). This arrangement has the disadvantage that weak signals will be rather distorted and that the load put on the source of anode current will vary widely with the amplitude of the signal. Strong signals, on the other hand, can be amplified without undue distortion, the straight portion of the valve characteristic being entirely available for the positive half waves only. 7

It is the object of my present invention to have an amplifier of the push-pull, or balanced, type so arranged that the advantages obtained by the class A and B amplifiers are combined.

According to the invention means are provided whereby the working point on the valve characteristics is automatically shifted from a point on the straight portion of the anode current-grid bias characteristic towards the cut-off point on said characteristic as the amplitude of the signals increases.

According to the invention this may be done by causing an adequate increase of the grid bias of the balance connected valves. Consequently, the amplifier will behave as one of the class A type when the signals are weak, and as one of the class B type when the signals are strong.

The automatic adjustment of the grid bias in dependency of the strength of the signal may be obtained by using the voltage drop across a resistance in the anode circuit, the anode current and consequently the said voltage drop increasing with the strength of the signal after the latter will have surpassed a certain predetermined value. I

The same result may however obviously be obtained in various other ways, e. g. by rectifying part of the incoming waves either before or after amplification, and causing the rectified current to produce a voltage drop which in turn may be directly or indirectly used for biasing the grids of the balance connected valves. r According to another embodiment of the invention use is made of the fact that In consequence of the ohmic resistance of the anode circuit the anode potential will fall on increase of the signal, i. e., on increase of the mean anode current, so that the characteristic itself will be shifted, and,if the grid bias is kept unaltered, the working point will gradually creep nearer. to the cut-off point on increase of the amplitude of the signal, and vice versa.

- In accordance with this part of the invention the ohmic resistance of the source of anode current, usually a so-called battery-eliminator,..is so chosen, that the voltage drop obtained thereby is about just sufficient to make the working point shift from the class A to the class B point when the amplitude of the signal varies from a minimum toa maximum.

In practice I have found that the ohmic redesired. This may be done in various ways, andso that the-two effects either sustain or counteract each other.

From the foregoing it will be clear that the correct working of any thermionic amplifier will be dependent to a larger or smaller degree on the anode voltage as a function of the load put on the source of anode current. This factor will be of special importance when the valves are work-' ing at or near the cut-ofi point of their charac teristics. From this it follows that it is essential to have the said function well in hand, which, in the majority of cases will mean that the anode voltage drop at full load should be kept within a V certain limit.

When dry cells, accumulator batteries or. gen-,

above condition will in general not provide any difficulty.

When, however, battery eliminators are used, comprising rectifying and filtering elements, the internal resistance will in general be too large, unless the apparatus is made unreasonably large, heavy and expensive.

According to a further feature of this invention this difficulty may be overcome by so designing the battery eliminator that its apparent resistance decreases on increase of theload and vice versa.

This may be realized by the use of an iron core choke, which is nearly saturated at the normal value at the direct current load. Wherever the expression battery eliminator is used in the foregoing and following, any device other than a battery or generator and capable of supplying a more or less smooth direct current, such as required in the circuits of thermionic valves, is to be understood thereby.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claim, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 is a circuit arrangement according to one embodiment of this invention;

Fig. 2 is a diagram showing the shift of grid bias in the circuit according to Fig. 1;

Fig. 3 shows a case, where the amplifier is fed by a battery eliminator of predetermined internal resistance;

Fig. 4 is a diagram showing the shift of the characteristic on variation of the load;

Fig. 5 shows a case where a reduced shift of the characteristic curve is combined with a reduced shift of the grid bias; and

Fig. 6 is the correspondingdiagram.

Referring to Fig. 1, the grids of the balance connected Valves V1 and V2 are biased by means of a battery C and by the voltagedrop occurring in the resistance R, through which the anode current is supplied by a battery B of comparatively small internal resistance, so that the potential at the anodes of the valves may be considered constant for all practical purposes.

The resistance R is so designed, that on increase of the anode direct current, e. g. as a consequence of an increase of signal strength from zero to maximum, the grid bias of the valves is increased so as to shift the working point from substantially the midpoint on the straight portion of the characteristic towards the cut-01f point, or very nearly so.

This is illustrated in Fig. 2, The grid bias on zero signal will have the value n, which is constituted by the algebraic sum of the voltage given by battery C and the voltage drop in the resistance R owing to the anode current flowing therein.

When signals of some appreciable amplitude are being impressed on the grids, the anode current will increase and so will the voltage drop in resistance R, thus causing an increase of the grid bias. Conditions are made such that when the signals have reached the maximum amplitude which can be amplified free of distortion, the grid bias has been increased by an, and the working point on the characteristic has been shifted from a at or near the middle of the straight, portion, to b near the cut-off point. With a symmetrically arranged amplifier of the type shown in Fig. 1, there is developed across the bias resistor R no voltage having the wave form of the original signal, because the grid of one tube becomes more positive by the same amount that the grid of the other becomes negative upon application of the signal voltage. There is, however, developed across resistor R some spurious alternating current voltage, which is due to the non-linearity of the tubes characteristic. This spurious alternating current voltage across R may be referred to as distortion because its cause is precisely what would cause distortion in a single tube system. But, only by way of explanation is the term distortion used. The distortion across R operates both grids in phase, and, therefore, does not appear in the combined output of the system.

Now the distortion across R contains a great many alternating current components when the wave form of the original signal is complex, and, whether said wave form is complex or not, the distortion across R contains a, direct, current component. This is one way of saying that the average voltage across R increases due to the application of the original signal voltage to the grids. This direct current component is persistent exactly the same way that the alternating current components persist. This direct current component can, of course, be read by connecting a direct current voltmeter across R, precisely the same as thealternating current components can be read by connecting an alternating current voltmeter across R. No condenser is necessary across R to maintain the direct current component across it, for the same reason that the alternating current components are maintained therein. All spurious components across R (and the direct current component is one of them) are maintained by the non-linearity of the tubes characteristic to I which the original signal voltage is applied. Not

only is a bypass condenser of capacity C shunted across R unnecessary, but such a bypass condenser is definitely a disadvantage because the time constant CR prevents the bias voltage 1 across R from properly following the changes in signal level. Due to a sudden increase in signal level which should cause the operating point to likewise quickly shift from point a. to 'point D, this shift of the operating point would become sluggish if a bypass condenser were connected across R.

According to Fig. 3 the push-pull connected valves V1 and V2 have their grids biased, to a constant value by the battery C only. The anode current is supplied by a so-called battery eliminator of a type too well known to need description. Apparatus of this class usually has a comparatively large internal resistance causing the voltage supplied to the anode to be materially dependent on the load. Any decrease of the anode voltage will cause the characteristic of the valve to be shifted to the right, and vice versa. Supposing the grid bias to remain unaltered this will mean a shift of the working point of the valve towards the lower band of the characteristic, and even past it, if the anode voltage will drop sufficiently. This will actually happen on the occurrence of strong signals, if a battery eliminator of the normal dimensions, as at present largely in use, is applied. According to the invention, the battery eliminator is so designed that its internal resistance will keep the drop of the anode potential within such limits that the working point will not pass the cut-off point of the characteristic, provided the signal strength will not pass a certain predetermined maximum.

In practice thi can be easily obtained by giving the series impedance of the filtering means F a comparatively small ohmic resistance. If said series impedance is a choke, as shown, this will mean an increase in size, weight and price as compared with the chokes usually applied for the same purpose. If the series impedance is an ohmic resistance, this resistance should be kept lower than usual, the capacity of the smoothing condensers being increased proportionally. In this case, too, there will be an increase in size, weight and cost.

The result obtained is graphically shown in Fig. 4. The grid bias supplied by battery C is constantly kept at a value it. When the signal increases from zero to maximum, the anode current-grid bias characteristic is shifting laterally over a distance g1 from the curve P1 to P2. The working point; is consequently shifted from the point a on P1 to b on P2.

Fig. 5 shows a way of decreasing the apparent internal resistance of a battery eliminator, leaving the usual dimensions of such device unaltered.

This is done by the use of a coil S, the iron or other magnetic core of which is near saturation for the average value of the direct current load. On the increase of such load, the inductance of the coil S will obviously decrease, so that the input voltage of the filter F will become larger.

This effect obviously counteracts the drop in the anode voltage by the ohmic resistance of the battery eliminator on the increase of the load. It is easy to see that this balancing efiect can be so proportioned that it is just sumcient to produce the working conditions diagrammatically shown in Fig. 4. In this case the said balancing effect of the coil need only be comparatively small.

In general, however, it will be preferable to have a high anode voltage available when the load is heavy, so that a substantial balancing action of the coil S will be useful. On the other hand the coil S will never be able to completely out-balance the increase in ohmic voltage drop on increasing load, so that the method of grid bias shifting described with reference to Figs. 1 and 2 may be advantageously combined with the introduction of the coil S. For this reason the resistance R is connected in series with the biasing battery C.

Fig. 6 shows diagrammatically, that the characteristic curve P1 is shifted laterally over a small distance Y2 to the curve P2 when the signal increases from zero to maximum. At the same time the grid bias n is increased by m, which represents the entire voltage drop in the resistance R on maximum load. In consequence of these two coinciding phenomena the working point is removed from the point a on the straight portion of the curve P1 to the point 1) near the cut-off point on the curve P2.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claim.

I claim:

In a push-pull amplifier including apair of vacuum tubes each having a curved characteristic relating plate current to grid potential, a signal input circuit, a source of fixed bias for the grids of said tubes which is wholly independent of the plate current of said tubes and a resistive impedance common to the space current paths of said tubes for supplying bias dependent upon plate current, an improvement for' causing operation of said amplifier to change over automatically from that of a substantially class A amplifier to that of a substantially class B amplifier as the strength of input signals impressed upon the signal input circuit increases from zero to a predetermined maximum, said improvement being characterized by said bias source and resistive impedance having relative values such as to produce a total bias in the absence of input signals which is sufficient to bring the operating point of said tubes onto a substantially linear portion of said characteristic but below the point of inflection thereof whereby increasing signals produce increasing plate current, and said resistive impedance having a magnitude such that the relative values of said two bias voltages produce in the presence of maximum input signals a total bias sufiicient to bring the operating point of said tubes down close to the cut-off point of said characteristic.

ANTON LEENDERT TIMMER. 

